Japanese Patent Laying-Open No. 2003-111203 discloses a drive gear for an automotive dynamo-electric machine that can suppress voltage fluctuations of the power source caused by the operating condition of the power-fed automotive dynamo-electric machine being switched. This drive gear for an automotive dynamo-electric machine includes a high-voltage power source system transferring power bidirectionally with an automotive dynamo-electric machine of high voltage via an inverter, a low-voltage power source system with a battery of low voltage generating voltage lower than that of the high-voltage power source system, a DC-DC converter arranged between the high-voltage power source system and low-voltage power source system to transfer power bidirectionally between the power source systems, and a control unit controlling the switching element in the DC-DC converter based on PWM (Pulse Width Modulation).
In the drive gear for an automotive dynamo-electric machine, the control unit feedback-controls the duty ratio of the switching element in the DC-DC converter such that the voltage of the high-voltage power source system converges to a predetermined target range. When the operating state of the automotive dynamo-electric machine switches between a power running operation and a regenerative operation, the control unit switches the maximum duty ratio of the switching element of the DC-DC converter such that the voltage and/or current of the low-voltage power source system comes within the battery tolerable range of the low-voltage power source system.
In other words, when the operating state of the automotive dynamo-electric machine is switched, the voltage/current of the low-voltage power source system will suddenly change in response to the quick change of the circuit state caused by the operating state switching, leading to the possibility of adversely affecting the battery of the low-voltage power source system and/or other components in the low-voltage system. Therefore, in the event of switching the transmission direction of the DC-DC converter in the drive gear for an automotive dynamo-electric machine, the maximum duty ratio of the switching element in the DC-DC converter is changed at the same time such that the voltage and/or current of the low-voltage power source system stays within the battery tolerable range of the low-voltage power source system. Thus, voltage variation in the power source system involved in the switching of the operating state of the automotive dynamo-electric machine can be suppressed.
In recent years, hybrid vehicles are attracting great attention from the standpoint of saving energy and environmental problems that have become a grave concern. A hybrid vehicle employs a battery, an inverter, and a motor driven by the inverter as the power source, in addition to a conventional engine. Some hybrid vehicles are known to further include a generator that generates electric power using the engine power, and an up-converter that boots and supplies to the inverter the voltage from the battery.
When excessive rotation of the motor occurs by, for example, a failure at the transmission provided at the output shaft of the motor that generates the motive power of the vehicle in a load driving apparatus incorporated in such a hybrid vehicle, control is effected to rapidly reduce the output torque of the motor in order to prevent motor damage. A sharp decrease in the motor output torque will cause a sudden decrease in the motor power consumption, whereby the demand-supply balance between the power consumed by the motor and the power supplied by the generator is temporarily disturbed. This may cause overvoltage (a voltage exceeding the breakdown voltage of the inverter) at the inverter input side.
Although the aforementioned Japanese Patent Laying-Open No. 2003-111203 discloses an approach to suppress voltage variation in the power source system, this approach may not prevent the overvoltage at the inverter input side that is caused by a sudden decrease in the motor torque, as set forth above. Since the voltage at the inverter input side begins to rise at the same time the motor torque begins to decrease abruptly, the approach disclosed in Japanese Patent Laying-Open No. 2003-111203 may not be able to suppress the voltage increase that occurs during the period starting from the sudden decrease in motor torque until switching the transmission direction of the converter. There is the possibility that occurrence of overvoltage cannot be prevented.